Gmail Growl is a free application that monitors your Gmail account for new messages and displays desktop notifications whenever you get a new message. Now with Prowl support, Gmail Growl will also forward said alerts as push notifications to your iPhone. All you need to do is:

1) Download and install Growl for Windows if you haven't already.
2) Download and install Gmail Growl 1.5.
3) Sign up for a Prowl account here if you haven't already.
4) Enter your Prowl username and password into Growl for Windows.
5) Give Gmail Growl your Gmail login info and set it to run with Windows.
6) Enjoy your push notifications.

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According to the download page, Gmail Growl 1.5 uses IMAP IDLE to keep an eye on your new Gmail messages, which provides near-instantaneous alerts of new messages—meaning Prowl's push notifications via Gmail Growl should be about as close to true push as you can get with Gmail on the iPhone right now.

Gmail Growl is freeware, Windows only. Prowl, on the other hand, will set you back $3 on the iTunes App Store.